domingo, 28 de diciembre de 2014

haproxy as a very very overloaded sslh

After using haproxy at work for some time I realized that it can be configured for a lot of things, for example: it knows about SNI (on ssl is the method we use to know what host the client is trying to reach so that we know what certificate to present and thus we can multiplex several virtual hosts on the same ssl IP:port) and it also knows how to make transparent proxy connections (the connections go through haproxy but the ending server will think they are arriving directly from the client, as it will see the client's IP as the source IP of the packages).

With this two little features, which are available on haproxy 1.5 (Jessie's version has them all), I thought I could give it a try to substitute sslh with haproxy giving me a lot of possibilities that sslh cannot do.

Having this in mind I thought I could multiplex several ssl services, not only https but also openvpn or similar, on the 443 port and also allow this services to arrive transparently to the final server. Thus what I wanted was not to mimic sslh (which can be done with haproxy) but to get the semantic I needed, which is similar to sslh but with more power and with a little different behaviour, cause I liked it that way.

There is however one caveat that I don't like about this setup and it is that to achieve the transparency one has to run haproxy as root, which is not really something one likes :-( so, having transparency is great, but we'll be taking some risks here which I personally don't like, to me it isn't worth it.

Anyway, here is the setup, it basically consists of a setup on haproxy but if we want transparency we'll have to add to it a routing and iptables setup, I'll describe here the whole setup

An example of a transparent setup can be found here but lacks some details, for example, if you need to redirect the traffic to the local haproxy you'll want to use the xt_TPROXY, there is a better doc for that at squid's wiki. Anyway, if you are playing just with your own machine, like we typically do with sslh, you won't need the TPROXY power, as packets will come straight to your 443, so haproxy will be able to get the without any problem. The problem will come if you are using transparency (source 0.0.0.0 usesrc clientip) because then packets coming out of haproxy will be carrying the ip of the real client, and thus the answers of the backend will go to that client (but with different ports and other tcp data), so it will not work. We'll have to get those packets back to haproxy, for that what we'll do is mark the packages with iptables and then route them to the loopback interface using advanced routing. This is where all the examples will tell you to use iptables' mangle table with rules marking on PREROUTING but that won't work out if you are having all the setup (frontend and backends) in just one box, instead you'll have to write those rules to work on the OUTPUT chain of the mangle table, having something like this:

Take that just as an example, better suggestions on how to know what traffic to send to DIVERT are welcome. The point here is that if you are sending the service to some other box you can do it on PREROUTIING, but if you are sending the service to the very same box of haproxy you'll have to mark the packages on the OUTPUT chain.

Once we have the packets marked we just need to route them, something like this will work out perfectly:

And that's all for this crazy setup. Of course, if, like me, you don't like the root implication of the transparent setup, you can remove the "source 0.0.0.0 usesrc clientip" lines on the backends and forget about transparency (connections to the backend will come from your local IP), but you'll be able to run haproxy with dropped privileges and you'll just need the plain haproxy.cfg setup and not the weird iptables and advanced routing setup.

Hope you like the article, btw, I'd like to point out the main difference of this setup vs sslh, it is that I'm only sending the packages to the ssl providers if the client is sending SNI info, otherwise I'm sending them to the ssh server, while sslh will send ssl clients without SNI also to the ssl provider. If your setup mimics sslh and you want to comment on it, feel free to do it.

3 comentarios:

Interestingly, OpenVPN TLS is "TLS over OpenVPN over TCP (or UDP)" and not "OpenVPN over TLS over TCP". I guess it works because req_ssl_sni does not need a TLS stream: it only needs to be able to find something which parse as a CLientHello anywhere in the buffer.

Is there an option to turn on SNI? I tried setting this up, but I don't see any servername indication coming through. I'm using tunnelblick, so maybe it's just that tunnelblick doesn't pass the correct option through to openvpn...